A MEMS device can be used for example as a microphone or as a loudspeaker. Such a MEMS device can be configured for example with a membrane and either one or two electrodes, which are also referred to as backplate(s) or backplate electrode(s). The membrane can be separated from the electrode or the two electrodes by (respectively) an air gap. Sound is converted into electrical signals by detection of the varying capacitance between the membrane and the electrode or electrodes if the membrane oscillates as a response to sound waves, or vice versa.
In the case of membrane sizes realized hitherto with a diameter in the range of 700 to 900 μm, a sufficient stiffness of the electrode(s) can be ensured by a construction of silicon nitride 140 nm/polysilicon 330 nm/silicon nitride 140 nm. For an increase in the membrane diameter, by means of which an improvement in a signal-to-noise ratio might be achieved, an increase in the stiffness of the electrode(s) might be necessary. The stiffness can substantially be defined by the two nitride layers having high intrinsic tensile stress.
For a further stiffening of the electrode(s), an increase in the layer thicknesses of the individual layers might be considered. This might have the following limitations: by way of example, a scaling of the layer thicknesses particularly of nitride may be possible only to a limited extent. In the case of LPCVD nitride used, a limit of the individual process technology may be reached in a region of a layer thickness of approximately 160 nm. In the case of some other process being employed, a maximum layer thickness of approximately 300 nm may possibly be achievable. An increase in the nitride thicknesses from 140 to 160 nm is considered to be insufficient for the stiffening of the scaled-up electrodes, and depending on, for example, a desired membrane size, a layer thickness of 300 nm can also possibly be insufficient for the stiffening of the scaled-up electrodes.
Even with a significant increase to 920 nm, a scaling of the polysilicon thickness exhibits only a relatively weak effect on the stiffening of the electrode. This can be deduced both on the basis of simulation results (see table below), and as a result of fabricating the system with a higher thickness of the polysilicon (990 nm).